EP4286675A1

EP4286675A1 - External combustion engine

Info

Publication number: EP4286675A1
Application number: EP22425024.1A
Authority: EP
Inventors: Davide Gentile
Original assignee: Innovative Technological Systems Srl
Current assignee: Innovative Technological Systems Srl
Priority date: 2022-06-01
Filing date: 2022-06-01
Publication date: 2023-12-06

Abstract

An external combustion engine comprises a first cylinder (11) and a second cylinder (12), in which a first piston (18) and a second piston (19) are suitable to slide, respectively, along a respective first (XI) and second axis (X2) which are parallel to each other, wherein the first (11) and the second cylinder (12) are fluidically connected to each other for the passage of a thermal carrier fluid suitable to cause the cyclic movement of the first piston (18) and of the second piston (19). The engine also comprises a drive shaft (20) rotating about an axis of rotation (Z), and with which crank means (25, 29) are integrally associated, the crank means (25, 29) being provided with at least a first pin (26) and at least a second pin (31), and first (21) and second (28) kinematic connection means suitable to connect respectively the first pin (26) and the second pin (31) to the first piston (18) and to the second piston (19), respectively, in order to provide, together with the crank means (25, 29), the rotation of the drive shaft (20).

Description

FIELD OF THE INVENTION

The present invention concerns an external combustion engine, also known as a Stirling engine, in which an isothermal expansion and compression cycle of a thermodynamic fluid, for example air, nitrogen, helium or other gases, is used to determine the alternating and cyclic movement of a displacer and a piston, so as to cause the movement of a determinate drive shaft from which to obtain mechanical work.

BACKGROUND OF THE INVENTION

External combustion engines are known, also known as Stirling engines, which exploit a temperature difference caused in a thermodynamic fluid, and carry out the cyclic and alternating movement of a displacer and a work piston.
The displacer and the work piston are kinematically connected to each other and to a drive shaft, which transmits the delivered power to a user machine.
In this type of known engine, it is therefore normally sufficient to cause a temperature difference in the thermodynamic fluid to start the functioning of the engine.
In particular, Stirling engines are known, with a so-called gamma or "γ" configuration, which comprise a first cylinder, a second cylinder, in which a first piston, also called a displacer piston, and a second piston, also called a work piston, respectively slide. The thermodynamic fluid flows freely from one cylinder to the other, remaining a single mass and allowing a separation between the heat exchangers associated with the displacer piston, and the space for compressions and expansions, associated with the work piston.
The displacer piston and the work piston are generally connected by respective connecting rods in proximity to a single crank pin. The latter is keyed onto a drive shaft from which the mechanical work obtained is taken.
The first cylinder is provided with a hot part disposed in proximity to the head of the latter or, in other words, in proximity to the upper dead center of the displacer, and with a cold part disposed in proximity to the lower dead center of the displacer. The hot part and the cold part of the first cylinder are respectively heated and cooled to transfer heat to the thermodynamic fluid contained in the first cylinder.
The hot part and the cold part of the first cylinder are suitably connected fluidically to each other, for example by providing passages between the external liner of the first cylinder and the displacer itself.
The first cylinder, in proximity to its cold part, is provided with a duct for connection with the head of the second cylinder so as to create a fluidic connection between the first and second cylinders.
By exploiting the expansion of the thermodynamic fluid, due to the input of heat from the hot part, the second piston, that is, the work piston, moves toward its lower dead center. The displacer moves toward the cold part, causing a cooling of the previously heated thermodynamic fluid and therefore causing a contraction of the fluid which draws the second piston toward its upper dead center.
The alternating displacement of the second piston, that is, the work piston, from the upper dead center to the lower dead center causes the rotation of the drive shaft and therefore the generation of mechanical work.
Normally, in known Stirling engines, the law of motion of the displacer is linked to the stroke of the shaft and the center distance of the corresponding connecting rod.
This configuration, however, entails a low efficiency of the Stirling-type external combustion engine which, in particular in correspondence with the upper dead center, entails zero speed, and therefore a slowdown in the movement of the displacer.
Moreover, this type of engine, despite having characteristics of silence, low environmental impact and reduced maintenance, provides a low compression ratio and has dead volumes in the cylinders, given by the fluid that expands and compresses without, however, generating useful work, and which lead to a loss of efficiency and performance.
In fact, Stirling engines with a traditional configuration do not allow to follow the ideal thermodynamic cycle, which provides, on a pressure-volume graph, two isochoric transformations and two isothermal transformations; in particular it is not possible to obtain isochoric transformations due to the dead volumes, therefore reducing the usable work area.
These limitations have prevented extensive use of engines of the type in question.
One purpose of the present invention is to provide an external combustion engine which is compact, simple to manufacture, efficient, and economical.
Another purpose of the present invention is to provide an external combustion engine that allows to improve the efficiency of known engines, in particular in correspondence with the upper dead center of the displacer.
Another purpose of the present invention is to provide an external combustion engine which allows to obtain a thermal cycle close to that of an ideal Sterling cycle, so as to obtain greater efficiency and performance compared with the state of the art.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

SUMMARY OF THE INVENTION

The present invention is set forth and characterized in the independent claim. The dependent claims describe other characteristics of the present invention or variants to the main inventive idea.
In accordance with the above purpose, an external combustion engine comprises:

a first cylinder and a second cylinder, in which respectively a first piston, or displacer piston, and a second piston, or work piston, are suitable to slide along respective work axes, and in which the first and second cylinders are fluidically connected to each other for the passage of a thermal carrier fluid suitable to cause the cyclic movement of the first piston and of the second piston;
a drive shaft rotating about an axis of rotation, and with which crank means are integrally associated which comprise a first and a second crank, which are respectively provided with a first pin and with a second pin which have pivot axes parallel to each other and disposed, furthermore, distanced from the axis of rotation; and
first and second kinematic connection means suitable to connect respectively the first pin to the first piston and respectively the second pin to the second piston, in order to provide, together with the crank means, the rotation of the drive shaft.

In particular, the first cylinder is provided with a hot chamber and a cold chamber between which the first piston is provided, which is made to slide in the first cylinder as a result of the expansion/compression of the thermal carrier fluid which is caused by the heating/cooling of the hot and cold chambers.
According to a preferential embodiment, the first piston and the second piston are sliding, inside the first cylinder and the second cylinder, respectively along a first axis and along a second axis, which are disposed parallel to each other.
According to one aspect of the present invention, the first axis of the first piston is offset with respect to the axis of rotation by a determinate distance.
In particular, the first connection means comprise a first connecting rod connected to the first pin, the free end of which is constrained to slide along an operating axis that is parallel and offset with respect to the axis of rotation.
The first connection means also comprise a rod connected to the displacer piston, the rod being constrained to slide along the first work axis, and a spacer element, to which the free end of the first connecting rod and the rod are connected in respective connection points which are distanced from each other in a direction that is transverse to the operating axis and to the first work axis by a defined offset distance.
In this way, the law of motion that regulates the movement of the piston is modified, allowing to increase the stroke and speed of the first piston, or displacer piston, in quadrature, that is, in correspondence with the respective lower and upper dead centers, and consequently increase the respective isochoric compression and expansion of the real Stirling cycle. The increase in isochoric compression and expansion allows to increase the area covered by the cycle in the Pressure-Volume (PV) plane and consequently the work that is done.
According to some embodiments, the connection points of the rod and of the connecting rod on the spacer element are distanced from each other by a segment that has a length that is coherent with a desired offset distance between the operating axis and the first axis, so as to offset the first work axis of the first piston with respect to the center of the crank means and therefore to the axis of rotation thereof.
According to some embodiments, the spacer element is configured in such a way as to allow the variation and adjustment of the length of the segment between the two connection points.
In this way, it is possible to adapt the respective isochoric expansions and compressions as a function of requirements, making the motor suitable to be used for different applications and with different thermal jumps, such as for example in cogeneration or recovery of waste heat originating from various sources (for example internal combustion).
According to possible solutions, the spacer element can have a telescopic conformation, or comprise two elements that are sliding relative to each other and can be clamped in stable reciprocal positions.
According to other embodiments, the second pin is offset by 90° around the axis of rotation with respect to the first pin.
According to other embodiments, the radial distance of the first pin with respect to the axis of rotation can be greater than the radial distance of the second pin with respect to the axis of rotation. This determines a greater stroke of the first piston inside the chamber compared to that of the second piston, thus allowing to have a greater quantity of work fluid that participates in the heating/cooling inside the first cylinder, that is, to increase the useful power that can be obtained from the thermodynamic cycle of the engine.
According to some embodiments, the second pin is offset by 90° with respect to the first and/or third pin around the axis of rotation. In this way, an angular offset of about 90° is achieved between the work piston and the displacer piston, which allows to improve the overall efficiency of the engine.
According to some embodiments, the first and second cylinders, with the respective first and second pistons, the drive shaft and the crank means constitute a drive unit.
According to other embodiments, the engine comprises two drive units, each having respective first and second cylinders with the respective first and second pistons, disposed sliding inside them, and two drive shafts, each provided with respective crank means to which the respective first pistons, or displacer pistons, and second pistons, or work pistons, are connected.
The "twin cylinder" version with two displacer pistons and two work pistons allows to increase the efficiency of the combustion engine compared to the version with single cylinder and single work piston.
In fact, while in the case of the single work piston a variable pressure given by the displacer acts on it, but substantially a fixed volume remains underneath it, so that in the compression phase it compresses the volume and in the expansion phase it expands it, substantially doing "useless" work, in the version with two work pistons, since they work in phase opposition, when one goes down, the other goes up, so the overall volume remains constant, and no useless work is done, thus minimizing pumping losses. This therefore allows a further improvement in the overall performance and efficiency of the combustion engine according to the invention.
According to some embodiments, the two drive shafts are counter-rotating, that is, they rotate in opposite senses to each other.
According to some embodiments, each of the first pistons, or displacer pistons, is kinematically connected to both the drive shafts by means of respective connecting rods and respective crank means, while the second pistons, or work pistons, are connected to a single respective drive shaft.
Therefore each first piston, or displacer piston, contributes to the movement of both work pistons.
According to some embodiments, along each drive shaft, the second connection means of the work pistons are positioned in an intermediate position between the first connection means of the displacer pistons.
According to some embodiments, the four cylinders are substantially disposed at the vertices of a quadrilateral, with the two work pistons aligned in a first direction and lying on a common lying plane, and the respective displacer pistons aligned in a second direction, orthogonal to the first direction.
According to other embodiments, the work axis of the respective displacer pistons has an offset with respect to the operating axis of each connecting rod and to the axis of rotation, which is adjustable according to the length of the spacer element that connects the ends of the two first connecting rods that move each displacer piston.
According to the embodiments that provide two drive units, on each drive shaft there are crank means comprising at least a first crank provided with a first pin for the connection of a displacer piston, a second crank provided with a second pin for the connection of a respective work piston, and a third crank provided with a third pin for the connection of the other displacer piston, wherein the second wheel is located in an intermediate position between the first crank and the third crank.
According to some embodiments, the first and third pins are offset by 180° with respect to each other around the axis of rotation.
In this way, it is possible to optimize the efficiency of the thermodynamic cycle of the engine and, in particular, to optimize both the mechanical work exchanges of the second piston, or work piston, and also the thermal exchanges inside the first cylinder, releasing the thermodynamic operating modes of the first piston with respect to the second piston.
Furthermore, both the particular angular offset configuration of the first, second and third pin, and possibly also the differentiation of the strokes of the pistons, allow to create a kinematic mechanism that is simple to manufacture and in which unfavorable kinematic conditions, for example caused by the dead centers of known crank and connecting rod mechanisms, are prevented, also allowing to increase the speed of movement of the work piston(s) in quadrature.

DESCRIPTION OF THE DRAWINGS

These and other characteristics of the present invention will become apparent from the following description of some embodiments, given as a non-restrictive example with reference to the attached drawings wherein:

fig. 1 is a lateral view from a first side of an external combustion engine according to some embodiments described here;
fig. 2 is a lateral view of the combustion engine of fig. 1 from another side;
fig. 3 is a lateral view from a first side of a combustion engine according to other embodiments of the present invention;
fig. 4 is a lateral view of the external combustion engine of fig. 3 from another side;
fig. 5 is a three-dimensional view of an external combustion engine according to other embodiments, in which the first and second cylinders have been removed to show the elements below;
fig. 6 shows, by way of example, the comparison of the course of the stroke of the work piston of a traditional combustion engine and of a combustion engine according to the invention.

To facilitate comprehension, the same reference numbers have been used, where possible, to identify identical common elements in the drawings. It is understood that elements and characteristics of one embodiment can be conveniently incorporated into other embodiments without further clarifications.

DESCRIPTION OF SOME EMBODIMENTS

We will now refer in detail to the possible embodiments of the present invention, of which one or more examples are shown in the attached drawings. Each example is supplied by way of illustration of the invention and shall not be understood as a limitation thereof. For example, one or more characteristics shown or described insomuch as they are part of one embodiment can be varied or adopted on, or in association with, other embodiments to produce another embodiment. It is understood that the present invention shall include all such modifications and variants.
With reference to the attached drawings, an external combustion engine, also known as a Stirling engine, is indicated as a whole with reference number 10 and comprises a first cylinder 11 and a second cylinder 12 which develop axially along a first X1 and a second axis X2 respectively, which are disposed in a fixed position and are parallel to each other.
The first cylinder 11 comprises a first part, or hot chamber 13, in proximity to its head, which is suitably heated by means of heating means 15, and a cold part, or cold chamber 14, which is cooled, for example by thermal exchange with a refrigerant fluid which is made to flow in a cooling channel created in the liner of the first cylinder 11.
According to some embodiments, the heating means 15 can comprise a tube bundle heat exchanger 16 which is passed through by a thermal carrier fluid.
In other embodiments, not shown, the hot chamber 13 can be heated with a direct flame on the external part of the first cylinder 11, or by means of one or more thermal concentrators, for example with lenses, panels, mirrors.
Relatively high temperatures can be reached in the hot chamber 13, for example about 400 °C - 500 °C.
Furthermore, the cold chamber 14 can be cooled, for example by providing natural or forced convection finned coils that cover the external surface of the first cylinder 11 or of the second cylinder 12, and relatively low temperatures can be reached in it, for example about 60 °C - 110 °C.
In order to increase the thermal exchange surface of the cold chamber 14, it is possible to provide that its internal surface is provided with a plurality of cooling fins.
The first cylinder 11 can also comprise, internally or peripherally to its liner, a regenerator 17, for example made of porous metal material with a high thermal exchange capacity. The regenerator 17 therefore has efficient thermal exchange properties and it is sized so as to prevent high losses of pressure of the fluid.
In particular, the regenerator 17 prevents the hot chamber 13 and the cold chamber 17 from being fluidically short-circuited with respect to each other, allowing to obtain an excellent thermal exchange between the hot fluid and the cold fluid.
Inside the first cylinder 11 (figs. 1 and 2) there is disposed a first piston, or displacer piston 18, sliding along the first axis X1, while inside the second cylinder 12 there is disposed a second piston, or work piston 19, sliding along the second axis X2.
The second piston 19 and the second cylinder 12 define a work chamber 33 inside which the thermodynamic fluid expands/compresses.
The work chamber 33 of the second cylinder 12 and the cold chamber 14 of the first cylinder 11 are fluidically interconnected by means of a connection duct, not shown, through which the fluid present in the cold chamber 17 can pass as a result of the expansion/compression of the thermodynamic fluid.
Specifically, the connection duct can be connected to the second cylinder 12 in proximity to the head of the latter, and to the first cylinder 11 in proximity to the lower dead center of the displacer piston 18.
The external combustion engine 10 also comprises a drive shaft 20 disposed rotating on main journals, not visible in the drawings, about an axis of rotation Z.
Specifically, it is advantageous to provide that the axis of rotation Z is disposed substantially orthogonal with respect to the second axis X2. In fact, in this way it is possible to reduce both the overall dimensions of the engine, and also to distribute the inertial loads of the engine on the main journals in a more uniform manner.
According to some embodiments, the second axis X2 intersects the axis of rotation Y, while the first axis X1 is disposed offset with respect to it by a determinate distance D.
The first piston 18 is kinematically connected to the drive shaft 20 by means of first connection means 21.
The first 11 and the second cylinders 12 with the respective first 18 and second pistons 19, the drive shaft 20 and the first 21 and second connection means 28 can define a drive unit, or module 40.
According to some embodiments, the first connection means 21 comprise a rod 22 connected in a fixed manner with one of its ends to the displacer piston 18 and constrained to slide axially along the first axis X1.
The first connection means 21 also comprise a first crank 25 keyed to the drive shaft 20 and rotatable together with it about the axis of rotation Y, and a first connecting rod 24 pivoted with a first end 24a to the first crank 25 in proximity to a first pin 26 thereof that is radially distanced from the axis of rotation Z.
The first crank 25 is provided, on the side opposite to the one on which the first pin 26 is provided, with counterweights 27 which perform the function of flywheel during the cyclic movement of the pistons.
According to some embodiments, the first connection means 21 also comprise a spacer element 23 which, in a first connection point PI, is pivoted with a second end 24b of the first connecting rod 24 and, in a second connection point, is connected to the rod 22.
The second connection point P2 is offset and distanced with respect to the first connection point P1 in a direction orthogonal to the first axis X1.
In this way, the second end 24b of the connecting rod 24 is constrained to move on an operating axis Y parallel to the first axis X1 and offset with respect to the axis passing through the center of the first crank 25 by an offset segment O.
According to some embodiments, the spacer element 23 can have an oblong shape and be disposed in a direction orthogonal to the first axis X1, with the respective first and second connection points PI, P2 disposed aligned along an axis of longitudinal development thereof.
In particular, the second connection point P2 is located in correspondence with the first axis X1, while the first connection point P1 is offset with respect to it by an offset distance D-O, given by the difference between the distance D present between the first axis X1 and the axis of rotation Z and the offset segment O.
According to some example and non-limiting embodiments, the distance D can be comprised between 1 mm and 100 mm, while the offset distance D-O can be comprised between 0.1 and 0.9 times the distance D, as a function of the applications of the engine 10.
According to some example embodiments, the distance D can be comprised between 10-100 mm, while the offset distance D-O can be comprised between 1-80 mm.
The law of motion of the displacer 18, and consequently that of the work piston 19 connected to it through the rotation shaft 20, is therefore varied as a function of the value of the offset distance D-O.
Fig. 6 shows, by way of example, a graph of the position of the displacer 18 in the case of an engine of the traditional type, indicated with a broken line, and in the case of an engine 10 according to the present invention, indicated with a solid line.
The law of motion of the traditional engine is defined by the following formula (I): $S = \frac{C}{2} + I - \frac{C}{2} \cdot \cos (α) - \sqrt{I^{2} - \frac{C^{2}}{4} \cdot \sin {(α)}^{2}}$
where:

S is the space covered in the reciprocating motion of the displacer;
C is the stroke of the crank mechanism;
I is the center distance of the connecting rod;
α is the crank angle, in which zero is normally located in correspondence with the upper dead center.

The law of motion of the displacer 18 in the engine 10 according to the present invention is defined by the following formula (II): $S = \frac{C}{2} + \sqrt{I^{2} - {(D - O)}^{2}} - \frac{C}{2} \cdot \cos (α) - \sqrt{I^{2} - {(\frac{C}{2} \cdot \sin (α) - (D - O))}^{2}}$
where:

S is the space covered in the reciprocating motion of the displacer 18;
C is the stroke of the crank mechanism 25, 26;
I is the center distance of the first connecting rod 24;
α is the crank angle;
D-O is the offset distance.

According to some embodiments, the crank angle α can be considered equal to zero in correspondence with the upper dead center of the displacer piston 18.
By varying D-O, for example by modifying or replacing the spacer element 23, it is therefore possible to modify the law of motion, and to vary the speed of the displacer piston 18 in correspondence with the upper and lower dead center so as to reduce dead volumes to a minimum and bring the work cycle closer to that of an ideal Stirling cycle, increasing the useful work performed.
The second work piston 19 is connected to the drive shaft 20 by means of second connection means 28, which comprise a second crank 29 and a second connecting rod 30 pivoted with a first end 30a to a second pin 31 provided on the second crank 29.
The second pin 31 is radially distanced from the center of the second crank 29 and from the axis of rotation Z.
The first pin 26 and the second pin 31 respectively have a first pivot axis J and a second pivot axis K which are disposed substantially parallel with respect to each other and with respect to the axis of rotation Z of the drive shaft 20.
During the rotation of the first crank 25 and of the second crank 29, the first pin 26 and the second pin 31 are made to rotate about the axis of rotation Z of the drive shaft 20.
According to some embodiments, the external combustion engine 10 can comprise two drive units, or modules 40, 140, which are kinematically connected and cooperate with each other.
In particular, according to some embodiments, the external combustion engine 10 comprises two first cylinders 11, 111, two second cylinders 12, 112, in which respective first pistons 18, 118 and second pistons 19, 119 are suitable to slide, two drive shafts 20, 120 and first 21, 121 and second connection means 28, 128 which connect the first 18, 118 and second pistons 19, 119 to one and/or to the other of the drive shafts 20, 120.
The first 18, 118 and second pistons 19, 119 are able to slide along the respective first X1, X1' and second axes X2, X2', which are parallel to each other.
According to some embodiments, the first X1, X1' and the second axes X2, X2' are disposed at the vertices of a quadrilateral, for example a square, or a rhombus, in which the first axes X1, X1' are aligned in a first direction and lie on a common lying plane π, and the second axes X2, X2' are aligned in a second direction, which is orthogonal to the lying plane π (see for example fig. 5).
According to some embodiments, the drive shafts 20, 120 are rotating about respective axes of rotation Z, Z' parallel to each other.
According to some embodiments, the drive shafts 20, 120 are counter-rotating, that is, they rotate in opposite senses, one clockwise and one counterclockwise.
According to some embodiments, the first connection means 21, 121 connect each first piston, or displacer piston 18, 118, to both drive shafts 20, 120. In this way, both displacer pistons 18, 118 contribute to the movement of each work piston 19, 119.
According to some embodiments, the second connection means 28, 128 are configured to connect a respective second piston 19, 119 to only one of the drive shafts 20, 120.
According to some embodiments, the connection means 28, 128 each comprise a second connecting rod 30, 130, which can be connected to a second crank 29, 129 or be directly connected to the drive shaft by means of an articulation mechanism known in the state of the art as "gooseneck".
According to these embodiments, two spacer elements 23, 123 are provided, each provided with three connection points P1, P2, P3 aligned in succession to each other along their longitudinal development.
According to some embodiments, the first P1 and the third connection points P3 are symmetrical with respect to the second connection point P2. This allows to optimize the overall dimensions and the distribution of the loads of the combustion engine 10.
In accordance with some embodiments, the spacer elements 23, 123 can be of adjustable length, for example in a telescopic or sliding manner, in order to vary the distances between the connection points and therefore the offset distance DO.
According to other variants, it can be provided that the spacer elements 23, 123 are fixed, and that they can be removed and replaced with spacer elements 23, 123 of different lengths as a function of the speed desired for the displacer piston 18 in quadrature.
In accordance with the embodiments shown by way of example in figs. 3-5, the first connection means 21 comprise a pair of first cranks 25, 125 with respective first connecting rods 24, 124 pivoted to them with a first end 24a, 124a, and with the second end 24b, 124b to connection points PI, P3 of a first spacer element 23 which are opposite with respect to the first work axis X1.
In this way, each of the two ends 24b, 124b is constrained to slide on a respective operating axis Y, Y', wherein the two operating axes Y, Y' are parallel to each other and disposed in an intermediate position between the lying plane π and the axis of rotation Z, Z' of the drive shaft 20, 120 to which they are connected.
According to the embodiments of figs. 3-5, the first connection means 121 also comprise a pair of third cranks 34, 134 provided with respective third pins 35, 135 and a pair of third connecting rods 36, 136, connected with a first end 36a, 136a to the third pin 35, 135 and with the second end 36b, 136b to a second spacer element 123.
According to some embodiments, the first connecting rods 24, 124 and the third connecting rods 36, 136 are respectively symmetrical with respect to the plane π.
According to some embodiments, the second connection means 28, 128 are disposed, along each drive shaft 20, 120, in an intermediate position between the first connection means 21, 121, in particular they are disposed between respective first 25, 125 and third cranks 34, 134.
According to some embodiments, the first pins 26, 126 and the third pins 35, 135 are aligned along respective pivot axes J, L which are parallel to each other and to the axis of rotation Z.
According to some embodiments, the pivot axes J, L are angularly offset with respect to the axis of rotation Z by about 180°.
In this way, the two first pistons 18, 118 respectively connected to a respective spacer element 23, 123 work in phase opposition, so that when one goes down, the other goes up, and vice versa, thus optimizing the performance of the engine.
In other words, when one of the first pistons 18 is in correspondence with its upper dead center, the other one of the first pistons 118 is in correspondence with its lower dead center, and vice versa.
Since the pivot axes K, K' of the respective second pins 31, 131, to which the second pistons 19, 119 are connected, are angularly offset by 90° around the axis of rotation Z, Z' with respect to the pivot axes J, L, furthermore, the second pistons 19, 119 also work in phase opposition with respect to each other.
In this way, the volume of fluid of each module 40, 140 remains substantially unchanged, and therefore the dead volumes are reduced, allowing to obtain a work cycle close to an ideal Stirling cycle comprising two isothermal transformations and two isochoric transformations.
According to other embodiments, the first crank 25, 125, the second crank 29, 129, the third crank 34, 134, if present, and at least part of the connecting rods 24, 124, 30, 130, are contained within a containing casing, not shown, and they are suitably lubricated in an oil bath, providing, in a known way, an oil sump on the bottom of the containing casing.
It is clear that modifications and/or additions of parts may be made to the external combustion engine as described heretofore, without departing from the field and scope of the present invention.

Claims

External combustion engine comprising:
- a first cylinder (11) and a second cylinder (12), in which a first piston (18) and a second piston (19) are suitable to slide, respectively, along a respective first (X1) and second axis (X2) which are parallel to each other, said first (11) and second cylinder (12) being fluidically connected to each other for the passage of a thermal carrier fluid suitable to cause the cyclic movement of said first piston (18) and said second piston (19);

- a drive shaft (20) rotating about an axis of rotation (Z), and with which crank means (25, 29) are integrally associated, said crank means (25, 29) being provided respectively with at least a first pin (26) and at least a second pin (31), which have pivot axes that are parallel to each other and disposed, moreover, radially distanced from said axis of rotation (Z); and

- first (21) and second (28) kinematic connection means suitable to connect respectively said first pin (26) and said second pin (31) to said first piston (18) and respectively to said second piston (19) in order to provide, together with said crank means (25, 29), the rotation of said drive shaft (20), characterized in that said first axis (X1) is offset by an adjustable distance (D) with respect to said axis of rotation (Z).
External combustion engine as in claim 1, characterized in that said second axis (X2) intersects and is orthogonal to said axis of rotation (Z).
External combustion engine as in claim 1 or 2, characterized in that said first connection means (21) comprise a first connecting rod (24) pivoted with a first end (24a) to the first pin (26), the second end (24b) of which is constrained to slide along an operating axis (Y) that is parallel to the first axis (X1) and offset with respect to the axis of rotation (Z) by an offset segment (O).
External combustion engine as in claim 3, characterized in that said first connection means (21) comprise a rod (22) connected to said first piston (18), constrained to slide along said first work axis (X1), and a spacer element (23), to which said second end (24b) of said first connecting rod and said rod (22) are connected in respective connection points (P1, P2) which are distanced from each other by an offset distance (D-O) in a direction that is transverse to said operating axis (Y) and to said first axis (X1).
External combustion engine as in any claim hereinbefore, characterized in that said second pin (31) is offset by 90° around said axis of rotation (Z) with respect to the first pin (26).
External combustion engine as in any claim hereinbefore, characterized in that it comprises two first cylinders (11, 111) and two second cylinders (12, 112) in which respective first pistons (18, 118) and second pistons (19, 119) are able to slide along respective first (X1, X1') and second work axes (X2, X2') which are parallel to each other, two drive shafts (20, 120) rotating about respective axes of rotation (Z, Z') with which respective crank means are associated and first (21, 121) and second (28, 128) kinematic connection means suitable to connect respectively said first pistons (18, 118) and second pistons (19, 119) to said drive shafts (20, 120), wherein said first work axes (X1, X1') lie on a common lying plane (π) which is disposed in an intermediate position between said axes of rotation (Z, Z').
External combustion engine as in claim 6, characterized in that the first axes (X1, X1') and the second axes (X2, X2') are disposed at the vertices of a quadrilateral, wherein the first axes (X1, X1') are aligned in a first direction and lie on a common lying plane (π), and the second axes (X2, X2') are aligned in a second direction, orthogonal to said lying plane (π).
External combustion engine as in claim 6 or 7, characterized in that said axes of rotation (Z, Z') are parallel to each other and said drive shafts (20, 120) are counter-rotating.
External combustion engine as in any claim from 6 to 8, characterized in that said first kinematic connection means (21, 121) connect each first piston, or displacer piston (18, 118) to both the drive shafts (20, 120), and said second connection means (28, 128) connect each second piston, or work piston (19, 119) to one drive shaft (20, 120).
External combustion engine as in claim 9, characterized in that said second connection means (28, 128) are disposed, along each drive shaft (20, 120), in an intermediate position between said first connection means (21, 121).
External combustion engine as in any claim from 6 to 10, characterized in that it comprises two spacer elements (23, 123), each connected in correspondence with a second connection point (P2) with a rod (22, 122) of one of the first pistons (18, 118), and in correspondence with respective first and third connection points (PI, P3), opposite to said second connection point (P2), with a pair of first connecting rods (24, 124), or respectively with a pair of third connecting rods (36, 136), which are in turn connected to respective first pins (26, 126) or third pins (35, 135) of first (25, 125) or third cranks ( 34, 134).
External combustion engine as in claims 7 and 11, characterized in that the first connecting rods (24, 124) and the third connecting rods (36, 136) are respectively symmetrical with respect to the lying plane (π).
External combustion engine as in claim 11 or 12, characterized in that the two first pistons (18, 118) respectively connected to a respective spacer element (23, 123) are configured to work in phase opposition, so that when one of the first pistons (18) is in correspondence with its upper dead center, the other of the first pistons (118) is in correspondence with its lower dead center, and vice versa.
External combustion engine as in any claim from 11 to 13, characterized in that the first pins (26, 126) and the third pins (35, 135) are aligned along respective pivot axes (J, L), which are parallel to each other and with respect to the axis of rotation (Z, Z') and are angularly offset with respect to said axis of rotation (Z, Z') by approximately 180°.
External combustion engine as in claim 14, characterized in that said second pistons (19, 119) are connected to respective second pins (31, 131) that have respective pivot axes (K, K') that are angularly offset by 90° around the axis of rotation (Z, Z') with respect to said pivot axes (J, L).